Mega Evolution

Mega Pokémon are special evolutions of Pokémon introduced in earlier generations. Some Pokémon need special Mega Stones in order to evolve into their Mega forms, the second final forms. Also; Pokémon can only enter their Mega Evolution Form by using the Mega Stone unique to their Species. For example: To Evolve Blaziken into MegaBlaziken; Blaziken must be holding the Mega Stone Blazikenite.

Mega Evolution does more than just change a Pokémon's apperance, it can also change their: Type, Ability, and Stats. A good example of a Pokémon's Type changing after Mega Evolution is Ampharos; which after Mega Evolution becomes a Dual Type Electric/Dragon Pokémon.

Mega Evolution is only possible during a Battle, although it is unsure when it occurs in said Battle. After the Battle ends, the Pokémon returns to normal. Also you are limited to having only one Mega Evolution in a battle.

Trivia

Kangaskhan the only Pokémon that does not have an appearance change when it evolves into its Mega form. Instead, the baby in its pouch comes out to assisst in fighting.

Mewtwo actually has two Mega forms. The Mega form that's seen is based on what version of the game is being played. Basically, If you get Version X, then you recieve a different version of MegaMewtwo than what you would've recieved if you got Version Y.

Blaziken is the only Pokémon whose Mega form cannot be obtained though normal gameplay. Instead, an event will be held where you can recieve a Torchic holding the Blazekinite, Blaziken's Mega Stone. Soon after, you can evolve Torchic all the way to Blaziken and let Blaziken Mega Evolve by holding the Blazekinite.[1]